The present invention relates to an amplifier with a compressor and an expander function for positive as well as negative half-waves of ground symmetrical signals with the slope of the compression or expander characteristic being controllable for at least one half-wave polarity.
The best known representative of such an amplifier with a compressor and expander function is the so-called VCA (voltage controlled amplifier). If it were required only for half-waves of one polarity, it could be composed of a circuit which produces an output equal to the logarithm of the input signal with a subsequently connected circuit which produces an output equal to the exponential power of its input signal, i.e., an antilog circuit, (see Tietze, Schenk, "Halbleiter-Schaltungstechnik" [Semiconductor Circuit Technology], 2nd Edition, published by Springer-Verlag, 1971, pages 282 et seq.). However, with such a circuit arrangement care would have to be taken that the slope of the compression and/or expansion characteristic is controllable. These characteristics represent the curve of the amplifier output level as a function of the input level.
FIG. 1 is a basic circuit diagram of such an amplifier for an input signal, which may have both half-wave polarities, at its input E, with the gain of the amplifier not being controllable. Between its input terminal E and a center terminal M, the amplifier has an instantaneous compressor with an approximately logarithmic gain characteristic. The compressor is composed of an operational amplifier 1 having a series input circuit composed of a coupling capacitor C1 and an input resistor R1, and a pair of feedback diodes D1, D2 connected in antiparallel. With the output voltage of operational amplifier 1 rising linearly in magnitude, a superproportional current, i.e. an exponentially increasing current, is returned via the feedback diodes D1 and D2 to the input of operational amplifier 1 so that greater amplitude values are amplified less than smaller values in the compressor.
In the subsequent instantaneous expander, which is disposed between center terminal M and amplifier output terminal A and which includes a pair of antiparallel connected diodes D3 and D4 connected in series with the input of an operational amplifier 2 having a feedback resistor R2, conditions are reversed. That is, in the expander, the currents through the input diodes D3 and D4, which increase exponentially with linearly increasing input voltage of the expander, are amplified by an operational amplifier 2 and its feedback resistance R2 and produce a correspondingly amplified output voltage across output terminals A.
To realize a controllable gain for the amplifier, the diodes D1-D4 of FIG. 1 are each replaced in FIG. 2 by the emitter-collector path of a respective transistor T1 to T4. More specifically, the diodes D1 and D2 of the compressor of FIG. 1 are replaced by a pair of opposite polarity type transistors T1 and T2 which have their respective emitters connected to the output of the operational amplifier 1 via respective resistors R3 and R4 and have their collectors connected together and to the inverting input of the operational amplifier 1. A source of operating potential is connected across the series connected emitter-collector paths of the transistors T1 and T2. The diodes D3 and D4 of the expander of FIG. 1, in turn, are replaced by a pair of opposite polarity type transistors T3 and T4 which are of the same polarity type as the transistors T1 and T2 respectively and which have their emitters connected to the respective emitters of the transistors T1 and T2 and their collectors connected together and to the inverting input of the operational amplifier 2. In order to control the slope of the compression and expansion characteristics of both the compressor and the expander for both half-waves polarities, the bases of transistors T1 and T4 are connected to ground via a resistor R5 and to a terminal 3, while the bases of transistors T2 and T3 are connected to ground via a resistor R6 and to a terminal 4; an adjustable d.c. voltage is applied to the pair of terminals 3-4.
In an emitter-collector path as shown in FIG. 2, the current through the transistor also rises exponentially with the linearly increasing emitter-base voltage as long as the associated base-collector voltage remains constant. If the potentials at the pair of adjustment terminals 3-4 are initially kept constant, the potential differences between the constant base potentials, on the one hand, and the half-wave shaped, pulsating emitter potentials, on the other hand, produce collector currents as an exponential function of the base-emitter voltages.
Controlling the gain is now possible in that an adjustment value (adjustment voltage) Us is applied to the pair of adjustment terminals 3-4, thus permitting the slope of the collector current/base-emitter voltage characteristic of each transistor to be varied because a change in potential at the bases causes a change in the collector-base voltage. It must here be considered that the potential at the collectors is almost zero because the collectors are each connected with one of the inverted inputs (so-called virtual zero points) of the respective operational amplifiers 1 and 2. In detail, the gain control operates such that, for example, an increase in potential at adjustment terminal 3 and a reduction at adjustment terminal 4 adjusts transistors T3 and T4 to conduct better and transistors T1 and T2 to conduct less well, with the result that the degree of expansion in the expander (2, R2, T3, T4) increases and the degree of compression occurring in the compressor (1, R1, T1, T2) decreases, so that the overall gain between input terminals E and output terminals A increases.
When an amplifier according to FIG. 2, or another amplifier for symmetrical signals with a compressor and/or expander function, is employed, second order distortions or harmonics may occur at the output although such distortions actually should not occur. The cause of these distortions is due to differences in behavior, sometimes as a function of temperature, of the two branches with respect to positive and negative half-waves.
To reduce such distortions, it is known to use selected pairs of transistors T1-T2 and T3-T4 with the individual transistors of each pair being thermally coupled together. This technique, however, requires undesirable expenditures.
Another way to reduce distortions is disclosed in Federal Republic of Germany DE-OS No. 3,021,788. According to this reference, behind input terminal E of FIG. 2 of the present application, a parallel path branches off for the symmetrical signals. In this parallel path a phase inverter amplifier is followed by a further circuit as shown in FIG. 2 which is then again followed by a further phase inverter amplifier before the output signals of the parallel path are added to the signals at output terminal A of FIG. 2. If now in FIG. 2, for example, the positive half-wave were given less preferential treatment (due to a weaker gain for transistor T3 than transistor T4), thus producing second order distortions, the inversion in the parallel path would put the other half-wave (through the transistor corresponding to T3) at a disadvantage. The addition at the output terminal A balances out the disadvantaged different half-waves (and thus the preferential treatment of the respectively other half-waves), presuming again that sufficient thermal couplings are provided.